feat: use superior icon resolution from libcosmic

[ZeusCraft10]

Enables the 'desktop' feature for libcosmic and uses 'as_cosmic_icon()' for search results to match the Applications overview behavior. This ensures that icons resolved to absolute paths by the launcher service (or themed icons) are correctly loaded.

References pop-os/cosmic-epoch#2847

[jacobgkau]

This branch seems to introduce a build failure.

Output from a local build (also failing on the build server):

   Compiling cosmic-launcher v1.0.3 (/home/jacob/Work/cosmic-launcher)
   Compiling humantime v2.3.0
   Compiling apply v0.3.0
   Compiling unicode-width v0.2.2
   Compiling css-color v0.2.8
   Compiling unicode-truncate v2.0.1
   Compiling console-subscriber v0.5.0
   Compiling pop-launcher-service v1.2.7 (https://github.com/pop-os/launcher/#eead361c)
   Compiling tracing-journald v0.3.2
   Compiling switcheroo-control v0.1.0 (https://github.com/pop-os/dbus-settings-bindings#87c3c356)
warning: unexpected `cfg` condition value: `a11y`
   --> src/components/list.rs:302:11
    |
302 |     #[cfg(feature = "a11y")]
    |           ^^^^^^^^^^^^^^^^
    |
    = note: expected values for `feature` are: `console`, `default`, and `wgpu`
    = help: consider adding `a11y` as a feature in `Cargo.toml`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default

error[E0599]: no method named `size` found for struct `cosmic::widget::icon::Handle` in the current scope
   --> src/app.rs:541:46
    |
539 | / ...                   desktop::fde::IconSource::from_unknown(&**name)
540 | | ...                       .as_cosmic_icon()
541 | | ...                       .size(64)
    | |                           -^^^^ method not found in `cosmic::widget::icon::Handle`
    | |___________________________|
    |

For more information about this error, try `rustc --explain E0599`.
warning: `cosmic-launcher` (bin "cosmic-launcher") generated 1 warning
error: could not compile `cosmic-launcher` (bin "cosmic-launcher") due to 1 previous error; 1 warning emitted
error: Recipe `build-debug` failed on line 45 with exit code 101
make[1]: *** [debian/rules:28: override_dh_auto_build] Error 101
make[1]: Leaving directory '/home/jacob/Work/cosmic-launcher'
make: *** [debian/rules:8: build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2

[jacobgkau]

Not seeing any differences in icon quality with this change. (E.g. gThumb still has a very low-resolution/blurry icon... but also not seeing anything done worse.)

Regression testing passed:

Launcher

• All windows on all workspaces appear on launch
• Choosing an app on another workspace moves workspaces and focus to that app
• Launching an application works
• Typing text and then removing it will re-show those windows
• Search works for applications and windows
• Search works for COSMIC settings panels
• Open windows are sorted above applications (e.g. "firefox")
• t: executes a command in a terminal
• : executes a command in sh
• = calculates an equation
• Search results are as expected:
  • cal returns LibreOffice Calc first
  • pops returns Popsicle first
  • shop returns the COSMIC Store first

@ZeusCraft10 Just pinging in case you want to double-check that you did everything you intended with this PR? The diff before the last force-push was larger than it currently is. Is just adding the use cosmic::desktop; line really enough to make icons higher-quality? Do you have any examples of ones that are improved with this?