[KPropertiesDialog] Add Apply button to apply the change without closing the window

Tested when applied locally on top of the v5.70.1 tag.

I believe this is a standard feature for configure dialogs that hopefully doesn't really need much explaination...

The enabling/disabling of the apply button is currently based on a fairly simple heuristics. The dirty bit would seem like a good fit for that purpose but it seems that some plugins (file properties and destop file for example) set the bit even if no changes are made...