Enable mouse highlighting for resource spawner

[JShep1]

I just noticed I couldn't highlight/select the text in the resource spawner search bar which is a little inconvenient so I enabled it by setting selectByMouse to true.

Signed-off-by: John Shepherd [email protected]

[codecov]

Codecov Report

Merging #402 into ign-gazebo3 will increase coverage by 11.59%.
The diff coverage is 80.21%.

@@               Coverage Diff                @@
##           ign-gazebo3     #402       +/-   ##
================================================
+ Coverage        65.71%   77.30%   +11.59%     
================================================
  Files              127      205       +78     
  Lines             6238    11018     +4780     
================================================
+ Hits              4099     8518     +4419     
- Misses            2139     2500      +361     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/Link.hh	100.00% <ø> (ø)
include/ignition/gazebo/Model.hh	100.00% <ø> (ø)
include/ignition/gazebo/SdfEntityCreator.hh	100.00% <ø> (ø)
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
include/ignition/gazebo/gui/GuiEvents.hh	0.00% <0.00%> (ø)
src/SimulationRunner.hh	100.00% <ø> (ø)
.../plugins/component_inspector/ComponentInspector.hh	28.57% <ø> (ø)
src/gui/plugins/entity_tree/EntityTree.cc	9.89% <0.00%> (ø)
... and 171 more

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