[LV2] UIs for mono plugins

[Johannes Lorenz]

LMMS is currently trying to implement Lv2. Historically, LMMS has used 
stereo for all plugins. In order to allow mono plugins, e.g. for LADSPA, 
LMMS duplicates those mono effects, which leads to each control being 
shown twice in an *internal* UI (internal like with LMMS widgets), e.g.:
```
input gain left        |  input gain right
compression ratio left |  compression ratio right
output gain left       |  output gain right
```
Each pair of controls can be controlled independently, but they could 
also be linked (ctrl-drag one on another, or clicking a "link" LED) 
letting one control mirror the value of the pair's other control.

Now, Lv2 introduces *external UI* (external like the plugin defines the 
UI). This raises the question of how many UIs LMMS should display for 
mono effects:

* Should there be one UI for each side, or one for both, or should both 
options be possible?
* If two UIs make sense, should we allow a user to link controls by 
using the "touch" feature? I.e.:
   1. user presses a host button "link controls"
   2. host UI tells user: now touch a plugin control
   3. user touches plugin control
   4. host UI recognizes touch and next time one mono plugin's control 
changes it somehow updates the control for the other mono plugin