Add Plugin Selection (Select All, None) button

[tutorgaming]

Add plugins Select All and Select None Button for handy selection :)

[bluppfisk]

Hello @Tutorgaming , thanks for your PRs. I'll have a look at them when I find the time this week. In the meantime, I'd like to comment on this particular PR. I'd rather see a select/clear all toggle in the top left corner of the table rather than an additional pair of buttons. Is this something you think you could do? I feel it's closer to the GTK design language

[tutorgaming]

Actually, I've struggled for a while to implement a tick box on the top left of the TreeView T_T.

At that time, I gave up and decided to implement that two buttons instead. Do you have any ideas or hint to look for?

[bluppfisk]

closing due to inactivity. @Tutorgaming I've changed the code base quite a bit and more changes coming. I'll still welcome a PR in the future if it complies with the GTK design language.