Drop down button in ColorBar switches (open/close) the PalettePopup
Fix DropDownButton::m_button style (left-align text and expansive widget)
Minor fix in Tabs when close button in each tab is enabled

Anyway the close button doesn't work at this moment.
Fix Splitter::onLoad/SaveLayout() to support jguiscale() changes
PaletteView: Apply jguiscale() to initial m_boxsize
Do not show empty tabs area in Tabs widget
Add Widget::onSetColor()
Change the style of scroll bars in ColorBar
ColorBar: Show 8 columns by default
ColorBar: change default FG/BG colors to white and black (RGB)
Remove hardcoded value from main_window.xml for the color bar splitter
Avoid drawing text over widget borders in SkinTheme::drawTextString()
Merge branch 'dev' of git@github.com:aseprite/aseprite.git into dev