The titles of the tabs (artist; album; genres...) to be displayed as icons instead of words. It would take much less space and for me it it looks better.

Yes, it's a good idea, especially for translations which often result in longer words :(
Of course, a problem is finding icons for all the tabs.
I've implemented this with a new option for FilterPanes, currently named tabmode. It can be one of 3 modes :
- text : current behavior, text only
- icon : if an icon is defined for this tab, use only the icon with no text, else use the text
- both : display the text and the icon if available

I haven't committed it yet, I'm still not sure about 2 things :
- the default mode
- the name of the icon, currently it's gmb-tab-PAGEID where PAGEID is artist, album, ... I might use these icons for other things too

I've found some icons (from OpenClipArt) that you can use for the tabs. I'm sending all that are potentially convenient for you and I also made my own choice of icons, that I consider to be the best selection. All of the icons are vector in svg format which enables easy editing and color scheme changing. 

good thing to look for icons for the tabs...

a few of those a pretty obvious, others really aren't (imo). what does the genre-icon represent? i'm also not very sure about "last played"...

also: why use an icon that's a "male" person?

I've committed the changes I mentioned (much) earlier, so you can use your icons.
Simply copy your icons in ~/.config/gmusicbrowser/icons/ with names like :
gmb-tab-savedtree.svg
gmb-tab-album.svg
gmb-tab-artist.svg
gmb-tab-folder.svg
gmb-tab-label.svg
gmb-tab-rating.svg
gmb-tab-added.svg
gmb-tab-lastplay.svg

and edit your layout to add the option tabmode=icon or tabmode=both to all your FilterPanes (also called FPanes)

commit : http://git.gmusicbrowser.org/commit/3267d250e3

Maybe I'll add an option to change the tab mode from the tab option menu, but there already is a lot of options in there, so I don't know.

Thanks for the icons, I'll think about adding some to gmb, but it's not easy to have an homogenous and nice icon theme ...