Stop use !important styles in our css files

[ivy-lli]

Currently, there are various places in your css files where you use !important.
I know that this is not something new and has been in your code for a while (and I honestly was surprised that I didn't find this earlier), but if you do so, it can be really hard to overwrite those styles.

Findings:

• https://github.com/eclipse-glsp/glsp-client/blob/master/packages/client/css/glsp-sprotty.css#L197-L216
• https://github.com/eclipse-glsp/glsp-client/blob/master/packages/client/css/keyboard.css#L28
• https://github.com/eclipse-glsp/glsp-client/blob/master/packages/client/css/navigation.css#L20-L26