Added new method for getting seqno field for the pane, which indicates
changes that happened on the pane, storing and comparing this field in
lua config, helps with that task for identifying tabs, which are still
active with those, where activity has stopped already.
Developers for desktop env like Gnome have wrong impression, that one
clipboard is enough for everybody. There might be true for environments
like MacOS or Windows, but not under X11, where there are three
clipboards, where two of them are accessible using two different
keyboard shortcuts. And they "unified" all of the shortcuts, to the
common lowest denominator and make shoft-insert and ctrl-v behave
exactly the same: paste from the clipboard instead of primary selection,
which I found at least stupid, to be polite.
Note, that this patch will work for most apps using GTK2/GTK3 toolkits,
while there might be other software, which follows this nonsense like
Firefox, so even if it uses GTK underneath, it still breaks the shortcut
key.
