Ruler/Markers rewrite, part 3 of N

Add initial documentation on TS ruler

The "Arranging" part of the manual will need to be adjusted accordingly
or possibly revised entirely.

include/time-signature-ruler.html

@@ -0,0 +1,51 @@
+<p>
+  The Time Signature ruler allows changing the time signature within one
+  session. In the example below, the session starts with a time signature of
+  4/4, then switches to 3/4 at bar 5, then to 6/8 at bar 13. The division of the
+  ruler and the bars/beats vertical lines on the canvas updates accordingly.
+</p>
+
+<figure>
+  <img style="width:75%;" width="50%" src="/images/multiple-time-signatures.png"
+  alt="Multiple Time Signatures">
+  <figcaption>Multiple Time Signatures</figcaption>
+</figure>
+
+<p>
+  Ardour also supports using complex time signatures such as 11/8 or 27/32.
+</p>
+
+<p>
+  New time signature markers can be created in two ways:
+</p>
+
+<ul>
+  <li><kbd class="mod1n"></kbd>-clicking on the ruler</li>
+  <li>Right-clicking on the ruler, then selecting the
+  	<kbd class="menu">New Time Signature</kbd> menu item</li>
+</ul>
+
+<p>
+  In both cases a new window will open to specify time signature settings:
+</p>
+
+<figure>
+  <img style="width:50%;" width="50%" src="/images/new-time-signature-marker.png"
+  alt="New Time Signature dialog">
+  <figcaption>New Time Signature dialog</figcaption>
+</figure>
+
+<p>
+  The right-click menu for existing time signature markers provides two options:
+</p>
+
+<ul>
+  <li><kbd class="menu">Edit…</kbd> to open the
+  	<kbd class="menu">Edit Time Signature</kbd> dialog;</li>
+  <li><kbd class="menu">Remove</kbd> to delete the selected time signature marker.</li>
+</ul>
+
+<p>
+  The <a href="@@time-tempo-and-meter">Time, Tempo and Time Signature</a> section
+  provides more hands-on information on using this ruler in production.
+</p>

master-doc.txt

@@ -528,12 +528,19 @@ part: subchapter
 ---
 
 ---
-title: BBT markers
+title: Bars:Beats
 include: bbt-markers.html
 link: bbt-markers
 part: subchapter
 ---
 
+---
+title: Time Signature
+include: time-signature-ruler.html
+link: time-signature-ruler
+part: subchapter
+---
+
 ---
 title: Creating Location Markers
 include: creating-location-markers.html