Debloat side panel by reduce the size of titles

Every time I use okular, I feel my available screen is squeezed into a tiny region for me to view PDF files.

The table of contents is much shorter.

However, okular's title bar + toolbar is in fact much slimmer than similar products.

Then I realize my mini mental pressure comes from the thicc and heavy left panels.

The info "thumbnails" is presented 3 times -> thicc title + tooltip + icons

The problem is intensified by tabbar. Removing those arguably redundant labels will save a lot of space.

Actual usable height is wasted by the title "Thumbnails".

After the change

I think removing them won't introduce usability problems, since the icons are there and each side panel is different in both functionality and presentations.

They are probably nice for the first 10 seconds of using okular, but I think they are useless later on.

fix https://bugs.kde.org/show_bug.cgi?id=462321

Relevant art: pdf.js/firefox also have only icons + tooltips and I think it is OK.