[BUG] All dialogs/popups should have consistent buttons

[atextor]

How: List of performed steps

• Open various popups (details below)

What: Actual result vs expected result
Expected: All popups consistently have an X closing button, possibly also "OK" button to close
Actual: Some popups have an X, some have OK/Cancel, some don't...

Input (files/screenshots)

"Workspace" does have an X button:

But "SAMM Elements" doesn't:

"Settings" has Cancel / Apply / OK:

But "Notifications" does not:

(it should have an "OK" button to close)

Neither does the "Help" dialog:

(it should also have an "OK" button to close)

And the "Entity Properties Configuration" dialog has Cancel and Save:

[chris-volk]

Occured on 5.5.0 linux

[chris-volk]

• For all dialogs that currently don't have any button (e.g. help, notifications), add one primary button labelled "Close".
• Investigate why the Close button for SAMM Elements is missing (on my machine it's there, 5.6.0 windows)