theme: Don't force too large a minimum size of PopupSliderMenuItem.

This should be smaller, or removed entirely, but just making it
smaller will prevent too many side-effects on xlets relying on a
minimum width.

This keeps narrower popup menus from being forced larger when a
submenu is opened, which can be jarring. As an example, this occurs
in the right-click->Applications expander in the sound applet.

We use sliders in the power applet also, bump that up a bit from our
new minimum, it looks nicer.

Ref: linuxmint/mint21.2-beta#36

data/theme/cinnamon.css

@@ -205,7 +205,7 @@ StScrollBar StButton#vhandle:hover {
 }
 .popup-slider-menu-item {
  height: 1em;
- min-width: 15em;
+ min-width: 6em;
  -slider-height: 0.3em;
  -slider-background-color: #666666;
  -slider-border-color: #555555;

files/usr/share/cinnamon/applets/power@cinnamon.org/applet.js

@@ -206,6 +206,7 @@ class BrightnessSlider extends PopupMenu.PopupSliderMenuItem {
  this.removeActor(this._slider);
  this.addActor(this.icon, {span: 0});
  this.addActor(this._slider, {span: -1, expand: true});
+ this._slider.style = "min-width: 10em;";
 
  this.label = label;
  this.tooltipText = label;