DlgUnitsCalculator: some tweaks

- we call it "unit system" in the preferences, thus call it here so too
- add tooltips
- we have unit categories

src/Gui/DlgUnitsCalculator.ui

@@ -114,12 +114,17 @@ To add a calculation press Return in the value input field</string>
       <item row="0" column="2">
        <widget class="QLabel" name="label_6">
         <property name="text">
-         <string>Scheme:</string>
+         <string>Unit System:</string>
         </property>
        </widget>
       </item>
       <item row="0" column="3">
-       <widget class="QComboBox" name="comboBoxScheme"/>
+       <widget class="QComboBox" name="comboBoxScheme">
+        <property name="toolTip">
+         <string>Unit system to be used to the Quantity
+The preference system is the one set in the FreeCAD preferences</string>
+        </property>
+       </widget>
       </item>
       <item row="1" column="0">
        <widget class="QLabel" name="label_5">
@@ -130,6 +135,9 @@ To add a calculation press Return in the value input field</string>
       </item>
       <item row="1" column="1">
        <widget class="QSpinBox" name="spinBoxDecimals">
+        <property name="toolTip">
+         <string>Decimals for the Quantity</string>
+        </property>
         <property name="minimum">
          <number>2</number>
         </property>
@@ -144,7 +152,7 @@ To add a calculation press Return in the value input field</string>
       <item row="1" column="2">
        <widget class="QLabel" name="label_4">
         <property name="text">
-         <string>Units:</string>
+         <string>Unit Category:</string>
         </property>
        </widget>
       </item>
@@ -156,6 +164,9 @@ To add a calculation press Return in the value input field</string>
           <verstretch>0</verstretch>
          </sizepolicy>
         </property>
+        <property name="toolTip">
+         <string>Unit category for the Quantity</string>
+        </property>
        </widget>
       </item>
      </layout>

src/Gui/DlgUnitsCalculatorImp.cpp

@@ -50,7 +50,7 @@ DlgUnitsCalculator::DlgUnitsCalculator( QWidget* parent, Qt::WindowFlags fl )
     ui->setupUi(this);
     this->setAttribute(Qt::WA_DeleteOnClose);
 
-    ui->comboBoxScheme->addItem(QString::fromLatin1("System schema"), static_cast<int>(-1));
+    ui->comboBoxScheme->addItem(QString::fromLatin1("Preference system"), static_cast<int>(-1));
     int num = static_cast<int>(Base::UnitSystem::NumUnitSystemTypes);
     for (int i=0; i<num; i++) {
         QString item = QString::fromLatin1(Base::UnitsApi::getDescription(static_cast<Base::UnitSystem>(i)));