Remove "Create new database" from tool bar

It's used extremely rarely, having it in such a prominent position
in the tool bar isn't justified. Also, with the Material Design
icons, its tool bar icon can easily be confused with "create new
entry".

Fixes #475

src/gui/MainWindow.ui

@@ -356,7 +356,6 @@
    <attribute name="toolBarBreak">
     <bool>false</bool>
    </attribute>
-   <addaction name="actionDatabaseNew"/>
    <addaction name="actionDatabaseOpen"/>
    <addaction name="actionDatabaseSave"/>
    <addaction name="separator"/>