Change reveal password button logic

Summary:
Indicate state, not the action it will have. When the password field is empty it feels like the password will be shown once I start typing. Dolphin has been doing it this way for the "show hidden files" action forever.
Also only show the button when the field contains text or when revealing the password, like in the PolKit dialog.

Test Plan:
Empty field

Dolphin
Show hidden files, files are currently hidden

Hide hidden files, files are currently shown

Reviewers: Plasma, VDG, ngraham, broulik, ndavis

Reviewed By: VDG, ngraham, ndavis

Subscribers: ndavis, kde-frameworks-devel, rizzitello, elvisangelaccio, ngraham, starbuck, plasma-devel

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10362