Changed beamercolorbox parameters

Changes the box parameters from:
   leftskip=0.3cm,
   rightskip=0.3cm,
   ht=2.5ex,
   dp=1.5ex

to:
   sep=1.5ex

And added a strut to ensure short characters always make a minimum box
height of 4.5 ex if there are small (e.g. -) characters in the title
only.

beamerouterthememetropolis.dtx

@@ -203,12 +203,9 @@
   \nointerlineskip
   \begin{beamercolorbox}[%
       wd=\paperwidth,
-      leftskip=0.3cm,
-      rightskip=0.3cm,
-      ht=2.5ex,
-      dp=1.5ex
+      sep=1.5ex,
     ]{frametitle}
-  \insertframetitle%
+  \insertframetitle\rule{0pt}{1ex}%
   \end{beamercolorbox}%
   \vspace{\@metropolis@voffset}
 }
@@ -216,12 +213,9 @@
   \nointerlineskip
   \begin{beamercolorbox}[%
       wd=\paperwidth,
-      leftskip=0.3cm,
-      rightskip=0.3cm,
-      ht=2.5ex,
-      dp=1.5ex
+      sep=1.5ex,
     ]{frametitle}
-  \insertframetitle%
+  \insertframetitle\rule{0pt}{1ex}%
   \end{beamercolorbox}%
   \usebeamertemplate*{progress bar in head/foot}
   \vspace{\@metropolis@voffset}