Cleanup Plugin Manager dialog

Merge "Name" and "Description" columns, and move plugin details to
tooltips.
Try to fix some duplicated accelerators inside Edit->Commands
Update tooltip string for check_cmdline_new_files
Update open string on filebrowser plugin to make clearer, that the file will be opened in Geany
Hide column headers in plugin manager dialog

They are not very useful and waste both vertical (the headers
themselves) and horizontal (first column wider because of title)
screen space.
Merge branch 'plugin-dialog-cleanup'