Made tooltip on TileSet Paint Button give more information when disabled

[TheSofox]

Fixes #78162

When TileSet becomes read only (such as multiple scenes using it at the same time), the Paint Button becomes disabled. Apparently this behaviour is not properly communicated to users. This PR changes the tooltip of the Paint Button when enabled/disabled to communicate this behaviour better to the user. When enabled the tooltip is the normal "Paint properties.", however now when disabled the tooltip is "TileSet is in read-only mode. Make the resource unique to edit TileSet properties."

[groud]

Hey, thanks for the contribution!

I think the idea is quite good, and the implementation is acceptable, but it is a bit unusual to define strings as variables like that. It makes things a bit more difficult to track IMO. I think I would suggest, instead doing it that way, to create some sort of an update_readonly() method, and update buttons/tooltips/etc... there.
That would avoid having to define those strings elsewhere.

[TheSofox]

Good feedback, I've implemented it by adding a _update_buttons_enabled() function and removing the string variables.

[TheSofox]

Renamed function _update_buttons() since it just seemed neater.
The function also contains other bits of repeated code, so I think it cleans up the code overall.

[akien-mga]

Thanks!