shell: hide sidebar after closing find bar.
When the side-bar was closed before opening the find-bar it will close again. This also fixes the state of the toggle button not correctly resetting when closing the sidebar via ESC.
Closes #124 (closed)
I haven't done any gtk stuff in C before, so I don't know if the memory management here makes any sense. I'm also not sure if it makes sense to merge this when pps-window is being ported to rust, but I was having a look at the project an thought I try solve a newcommer issue to get a feel for it. So feel free to just close this if this is useless.
Edited by nee