Add support for entering single, double, and ending barlines

[martha-thomae]

Fix Issue #121

• single barline is entered with keystroke /
• double barlines with |
• and final barline with =

The table shows the encodings in Humdrum and MEI:

Type of Barline	Keystroke	Humdrum Encoding	MEI Encoding
single barline	/	= or =|	@form = single
double barline	|	=||	@form = dbl
final barline	=	==	@form = end

---

Proof:

Rendering of keystrokes: c a b / a b | a b =

Humdrum

**mens	**text
*clefC3	*
sA	.
sB	.
=|	.
sA	.
sB	.
=||	.
sA	.
sB	.
==	.
=-	=-
*-	*-
!!!RDF**kern: @ = marked note

MEI

<score>
  <scoreDef midi.bpm="600">
    <staffGrp symbol="bracket">
      <staffDef n="1" lines="5" notationtype="mensural.white" xml:id="triplum" clef.shape="C" clef.line="3">
        <label>triplum</label>
        <mensur/>
      </staffDef>
    </staffGrp>
  </scoreDef>
  <section>
    <staff n="1">
      <layer n="1">
        <note xml:id="m-0ee77b8f-558f-405d-80ed-b3097b885ecd" dur="semibrevis" oct="3" pname="a"/>
        <note xml:id="m-c3369498-dab3-429e-9099-ddd1c5dfcf03" dur="semibrevis" oct="3" pname="b"/>
        <barLine xml:id="m-d6f953a0-95f1-4973-b7dc-5f36d70dcc62" form="single"/>
        <note xml:id="m-e070d9ef-f145-4800-b752-7f67b4d76052" dur="semibrevis" oct="3" pname="a"/>
        <note xml:id="m-6ae9338c-e2b3-4425-abb3-f7dd6c7fb0b7" dur="semibrevis" oct="3" pname="b"/>
        <barLine xml:id="m-8e75a3af-e2cf-4ee8-93c9-e7eb25e664fa" form="dbl"/>
        <note xml:id="m-0db27652-e97a-41dc-b197-86fa41018185" dur="semibrevis" oct="3" pname="a"/>
        <note xml:id="m-53b20ef6-e55e-4aa6-a64c-dc7264c17f1e" dur="semibrevis" oct="3" pname="b"/>
        <barLine xml:id="m-9e59d659-ab7d-4560-a07d-d76e46424730" form="end"/>
      </layer>
    </staff>
  </section>
</score>