Polish Notifier Plasmoid's UI

Summary:
This patch implements the following changes to Discover's Update Notifier plasmoid:
- Improve the layout of the controls within the window
- Update the button text to reflect what it will actually do (the previous text was misleading since it did not in fact perform the updates; it just opened Discover)
- Update the tooltip text to be clearer
- Give the button an icon to match its plasmoid icon

Test Plan:
Before:
{F6121567}

After:
{F6122915}

Reviewers: apol, #discover_software_store, #vdg

Reviewed By: apol, #discover_software_store

Subscribers: fabianr, esedgh, broulik, abetts, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14238