applets/colorpicker: fix tooltips
Colorpicker sets custom tooltips for its buttons (pick and color value). However, these do no actually work while they are in a panel, only while the fullRepresentation popup is open. This is because the automatic tooltip generation from the compactRepresentation loader kicks in, sees that the plasmoid's own tooltip values are null, and "helpfully" replaces the intended custom values with default ones. Due to the specific timing involved with the multiple stacked ToolTipAreas, there is also a visual glitch as the user moves the mouse over the applet, where the intended content can briefly flash in for a fraction of a second.
Instead, set the plasmoid's thumbnail main and sub text to empty string; this prevents the automatic assignment of a default tooltip and resolves the glitchy appearance.