Remove unused ThemeStore.js#getImageSize function

This function was used only by GraphicsMixin, which has been removed in
RAP 3.0M1.

Change-Id: I15f3c25747643d4357a4cf56faf1e5da80157a11
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>
Enhance Scale theming

The Scale theming lacks some properties/states/styles available in other
similar widgets like Slider and Scrollbar. Add:

- HORIZONTAL/VERTICAL style to both Scale and Scale-Thumb
- hover state for the thumb
- background-image property (gradient) to Scale and Scale-Thumb

352186: [Theming] Enhance Scale theming
https://bugs.eclipse.org/bugs/show_bug.cgi?id=352186

Change-Id: I2895f742f87e5bd5e3f13dc0497e3406b6a878ee
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>
Fix JS error when setting element backgroundImage to null

RWTQuery.js#fixBackgroundImage extracts background image URL from "url"
function if exists. This function fails with null image.

Add a check (similar to fixBackgroundGradient) for non null image.

454428: Client Error using Form ImageHyperlink Widget
https://bugs.eclipse.org/bugs/show_bug.cgi?id=454428

Change-Id: Id4e6cc958b084093090b344be76c1598e8af5225
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>