GDB Memory viewer: port MemoryRangeSelector to QFormLayout/QDialogButtonBox

Summary:
Using QFormLayout & QDialogButtonBox results in less code and
UI following automatically more closer to platform settings,
like order of Ok/Cancel buttons, icons on buttons, text on buttons,
alignment of labels in form.

Reviewers: volden, KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4549