Mnemonics, some misc fixes and a fix to the markdown in the LSP hover popover

Merged Vanadiae requested to merge Vanadiae/gnome-builder:misc-fixes-2021-07-19 into master

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Using the Alt+S mnemonic for the sourceview symbol tree (with the mouse cursor out of the way) has a weird bug which doesn't happen with devhelp's tree view. It doesn't show all the cell renderers like it does when opening the popover with the mouse. Instead I need to hover the row for it to show correctly (but only this row is set to visible, not the entire tree view at a time). Anyway, I'm not going to block this MR on it since all those treeview/cellrenderers are going away when porting to GTK 4.

Edited by Vanadiae

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