[launchermonitor] Don't send modified updates for added/removed files

Avoids bogus "modified" updates when a file appears and vanishes again
(e.g. for temporary files that are created during package installation).
[launchermodel] addItemIfValid should return LauncherItem *

Allows us to check whether the launcher icon has really been added.
[launchermodel] Check for existing icon when adding .desktop file
Merge pull request #115 from thp/launcher-install-load-icon

Look for existing icons when adding a .desktop file, fix extraneous "modified" file updates