Minor string cleanup (#248)

- Simplified some strings to not include unnecessary markup.
- Fix translator comments so that they actually make it to glabels_C.ts file
      - Reconciled the style of these comments.
      - Added translator comments

glabels/ui/VariablesView.ui

@@ -66,7 +66,7 @@
      <item>
       <widget class="QPushButton" name="addButton">
        <property name="toolTip">
-        <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;Add variable&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+        <string>Add variable</string>
        </property>
        <property name="text">
         <string>Add</string>
@@ -76,7 +76,7 @@
      <item>
       <widget class="QPushButton" name="editButton">
        <property name="toolTip">
-        <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;Edit selected variable&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+        <string>Edit selected variable</string>
        </property>
        <property name="text">
         <string>Edit</string>
@@ -86,7 +86,7 @@
      <item>
       <widget class="QPushButton" name="deleteButton">
        <property name="toolTip">
-        <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;Delete selected variable&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+        <string>Delete selected variable</string>
        </property>
        <property name="text">
         <string>Delete</string>