morebits: port SimpleWindow from jQuery UI to plain JS

[siddharthvp]

Looks and behaves exactly like the existing UI. There is also not much difference in the number of lines of code. It turns out that using jquery.ui and applying our tweaks over it wasn't really making the code lesser than re-implementing the dialog from scratch.

It took more than a week to get all the rough edges straightened out. I think the result is cleaner and more maintainable than the previous version. In particular, the CSS no longer needs to use !important or use over-specific selectors or inherit values to override jquery.ui behaviour.

The only visible changes are:

• Resizing is only supported from the bottom-right corner (similar to CSS native resize). Earlier, it was supported from all four corners and four edges. We could do that if there's demand.
• The close button looks a bit different (but behaves similarly).

Any other differences from the previous UI should be reported as a bug.

Partially solves #2300.

[siddharthvp]

QA checklist:

• when dialog opens, it should be at the center of the window
  • it should be the center even when page has been scrolled (can be tested in timeless)
• dialog should be draggable
  • should not be draggable beyond the left and top edges of viewport
• dialog should be resizable
• titlebar and buttonpane should not vertically contract when dialog is resized
• the scrollbox (used in Tag, Welcome) should resize with the dialog
  • elements below the scrollbox should be visible without having to scroll
• the close button should remain visible at narrow widths
• dialog should not be obscured by vector-2022 sticky header or other skin components
• the initial height of the dialog should be min(given height in code, height of content)
• modal dialogs (used for Welcome previews) should prevent user interaction outside
• dialog should be in focus when it is opened
• dialog in focus should close when esc key is pressed
  • check with dvorak keyboard layout as well (no need, dvorak and qwerty have esc at the same place)
• dialog inputs should be tabbable (able to be focused with tab presses)
• inline elements in dialog should not get treated as block elements (should not appear on separate lines)
• when multiple dialogs are opened, clicking on a dialog should bring it to top over other overlapping dialogs
• test across Chrome, Firefox and Safari
• test across vector-2022, vector, monobook and timeless
  • test nothing regressed in minerva

[siddharthvp]

I've checked for all kinds of UI issues I can think of over the past two weeks. Let's see how this goes :/