perf: reduce repaints amount caused by selection

[kornes]

PR reduces amount of painting while not changing behavior/visuals of text selection

• removed selecting_ bool, only used by triple click selection, which forced layout on each click release.
• fixed broken SelectionItem != operator which worked like ==, only used by already removed selecting_ so no side effects
• moved triple click selection due to above changes
• changed ChannelView::setSelection to repaint/emit signal only if selection was updated

tested:

• all consumers of selectionChanged signal
• single / double / triple click selection

[github-actions]

clang-tidy made some suggestions

[Nerixyz]

Aside from the segfault, this PR works for me. You really notice the absence of re-layouts when selecting in debug builds 👍.

Nit: This PR reduces the amount of (re-)layouts performed when selecting, the widget is still repainted (about) 50 times per second.