Allow users to enable indexing by clicking button

Before there was the instruction to enable indexing by modifying some config file without mentioning the format/where it is located. Also the instruction did not seem to work, because the value is read from ~/.config/katerc.

This will make it far easier to enable indexing. The setting in the projectsplugin config works too, but that is not as discoverable.

Before:
After:

(I resized the window, no reason to have the full window width screenshotted for this message)

@ngraham @waqar

I remember that I was really confused by the previous message ;)