chore: use unique context labels for the widgets

The below patch added `sidebargradients.ui` and it seems like this
file was based off `sidebarcellappearance.ui` as some widgets had
the exact same contexts as in `sidebarcellappearance.ui`. This broke
the builds due to duplication of `msgctxt` in the `.pot` translation
files.

https://gerrit.libreoffice.org/c/core/+/192043
I90b4f5368a552eb9ba399337a39e79455ea99fe6

Change-Id: I0f8e24aa52a2a77b87b2b685d22d7474736927af
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/200053
Reviewed-by: Michael Stahl <michael.stahl@collabora.com>
Tested-by: Jenkins

chart2/uiconfig/ui/sidebarcolors.ui

@@ -26,7 +26,7 @@
             <child>
               <object class="GtkMenuToolButton" id=".uno:ChartColorPalette">
                 <property name="visible">True</property>
-                <property name="tooltip-text" translatable="yes" context="sidebarcellappearance|bordertype|tooltip_text">Allow to select a color palette to apply to current chart data series.</property>
+                <property name="tooltip-text" translatable="yes" context="sidebarcolors|bordertype|tooltip_text">Allow to select a color palette to apply to current chart data series.</property>
                 <property name="halign">start</property>
                 <property name="valign">start</property>
                 <property name="use-underline">True</property>
@@ -38,7 +38,7 @@
             </child>
             <child internal-child="accessible">
               <object class="AtkObject" id="colorpalettetype-atkobject">
-                <property name="AtkObject::accessible-name" translatable="yes" context="sidebarcellappearance|cellbordertype-atkobject">Border Style</property>
+                <property name="AtkObject::accessible-name" translatable="yes" context="sidebarcolors|cellbordertype-atkobject">Border Style</property>
               </object>
             </child>
           </object>