Replace TokenizedBuffer.setVisible with simpler .startTokenizing

We will still wait to tokenize a buffer until it is visible in *some*
tab, but we no longer enable and disable tokenization as different
editors become visible.

src/text-editor.js

@@ -953,8 +953,10 @@ class TextEditor {
 
   // Controls visibility based on the given {Boolean}.
   setVisible (visible) {
-    const languageMode = this.buffer.getLanguageMode()
-    languageMode.setVisible && languageMode.setVisible(visible)
+    if (visible) {
+      const languageMode = this.buffer.getLanguageMode()
+      if (languageMode.startTokenizing) languageMode.startTokenizing()
+    }
   }
 
   setMini (mini) {
@@ -4130,7 +4132,9 @@ class TextEditor {
     }
     const languageMode = this.buffer.getLanguageMode()
 
-    if (languageMode.setVisible) languageMode.setVisible(this.component && this.component.visible)
+    if (this.component && this.component.visible && languageMode.startTokenizing) {
+      languageMode.startTokenizing()
+    }
     this.languageModeSubscription = languageMode.onDidTokenize && languageMode.onDidTokenize(() => {
       this.emitter.emit('did-tokenize')
     })

src/tokenized-buffer.js

@@ -23,7 +23,7 @@ class TokenizedBuffer {
     this.regexesByPattern = {}
 
     this.alive = true
-    this.visible = false
+    this.tokenizationStarted = false
     this.id = params.id != null ? params.id : nextId++
     this.buffer = params.buffer
     this.largeFileMode = params.largeFileMode
@@ -255,15 +255,15 @@ class TokenizedBuffer {
     }
   }
 
-  setVisible (visible) {
-    this.visible = visible
-    if (this.visible && this.grammar.name !== 'Null Grammar' && !this.largeFileMode) {
+  startTokenizing () {
+    this.tokenizationStarted = true
+    if (this.grammar.name !== 'Null Grammar' && !this.largeFileMode) {
       this.tokenizeInBackground()
     }
   }
 
   tokenizeInBackground () {
-    if (!this.visible || this.pendingChunk || !this.alive) return
+    if (!this.tokenizationStarted || this.pendingChunk || !this.alive) return
 
     this.pendingChunk = true
     _.defer(() => {