GUI updates

[dominikbach]

Fixes various minor bugs and inconsistencies in GUI and help texts, including:

• add pspm_installer.m as helper function
• update default for pspm_bf_hprf_fc.m in line with publication
• regroup Matlabbatch menu entries in line with function logic
• remove bugs in pspm_check_model, pspm_glm,
• improve last trial duration in pspm_dcm_inv, channel display logic in pspm_display, plots in pspm_extract_segments and pspm_find_sounds, long channel descriptions in pspm_init, improve error handling in pspm_select_channels
• add GUI missing epochs selector and implement in SF
• fix overwrite handling in SF GUI

Also fixes #757 .

[teddychao]

I think this pull request is ok to me. I tested the importing features (related to changed functions) through the GUI and found my tests working. Only minor suggestions to the UI variables that can directly call settings.