bugfixes and better handling of header-cache files

I never get bored of fixing bugs. Also, the header-cache should be more failsafe now.- fixed incoming spam filter bug, using less threads leads to increased performance- #964416 and #964415: both selection handling bugs- replace filter/filteractions combobox with menu, showing the user all available items without the need to scroll- fixed tablemodel update notification in updateGUI() from MarkFolderAsRead command