EXT-7087 FIXED Added flashing icons for im tabs and hooked them up in code

Added new attribute image_flash to button.xml which sets an image used for button icon when button is flashing.
Pointer to this image is stored in member mImageFlash from LLButton and is used in LLButton::draw(). There are two
ways an image can flash now - by making changes in color according to flash_color attribute or by changing icon from
current to the one specified in image_flash. Second way is used only if the name of flash icon is different from
"FlashIconAbsent" which is there by default. First way is used otherwise. Used new selfmade orange icons for
flashing tabs.

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branch : product-engine