[composer] Finer step for arrow head line width

Author: nyalldawson

src/ui/composer/qgscomposerarrowwidgetbase.ui

@@ -17,7 +17,16 @@
    <property name="spacing">
     <number>0</number>
    </property>
-   <property name="margin">
+   <property name="leftMargin">
+    <number>0</number>
+   </property>
+   <property name="topMargin">
+    <number>0</number>
+   </property>
+   <property name="rightMargin">
+    <number>0</number>
+   </property>
+   <property name="bottomMargin">
     <number>0</number>
    </property>
    <item>
@@ -195,7 +204,10 @@
             <property name="suffix">
              <string> mm</string>
             </property>
-            <property name="showClearButton">
+            <property name="singleStep">
+             <double>0.200000000000000</double>
+            </property>
+            <property name="showClearButton" stdset="0">
              <bool>false</bool>
             </property>
            </widget>
@@ -215,7 +227,7 @@
             <property name="suffix">
              <string> mm</string>
             </property>
-            <property name="showClearButton">
+            <property name="showClearButton" stdset="0">
              <bool>false</bool>
             </property>
            </widget>