Limit SCL Console buffer size to 5M characters by default

Low watermark limit for the buffer size can be configured from the new
SCL / Console preference page. High watermark is always set to 100 80
character rows larger than the low watermark.

Buffer size limiting can also be disabled/enabled entirely from the same
preference page.

gitlab #104

Change-Id: I8be203e60ae49fce53c39a4d251d2575a64b2543